In recent years, expansion of transmission capacity has been needed because of rapid growth of communication traffic. An optical communication module is a key device for an optical network system, and the optical communication module requires downsizing and high speed as the system increases its high speed and capacity. Digital coherent communication using multi-value phase modulation as a means for solving increase of the capacity of an optical communication system is widely distributed, and a transceiver for the digital coherent communication requires downsizing with increase of the capacity of the system.
Patent literature 1 (PTL1) proposes the laser module which has a wavelength monitoring unit, monitors a wavelength of an optical signal, and keep it constant, as a technology realizing stability of the wavelength of the optical signal. As shown in FIG. 4, in the laser module of PTL1, a first thermo-module 102 and a second thermo-module 112 are disposed to be close to each other on a bottom face of a package 101. The first thermo-module 102 and the second thermo-module 112 are devices which can heat and cool a surface thereof depending on a value and a direction of a flowing current, and are composed of a Peltier element or the like.
A base 103 is mounted on the first thermo-module 102, and a semiconductor laser element 106 and a parallel lens 105 which converts a laser light outputted from a front end face of the semiconductor laser element 106 into a parallel light are further placed thereon. An isolator 104 which blocks a reflected return light from an optical fiber 121 side and a parallel lens 107 which converts a monitoring laser light outputted from a back end face of the semiconductor laser element 106 into a parallel light are mounted on the base 103.
A base 113 is mounted on the second thermo-module 112, and a prism 114 which splits the monitoring laser light outputted from the back end face of the semiconductor laser element 106 into two directions at a predetermined angle is further mounted thereon. A first optical detector 116 which detects one light split by the prism 114 and a second optical detector 117 which detects a light which passes through an optical filter 115 in the other light split by the prism 114 are further mounted on the base 113. The optical filter 115 is composed of Fabry-Perot etalon. The optical filter 115, the first optical detector 116, the second optical detector 117 and the like compose the wavelength monitoring unit of PTL1.
In the laser module of PTL1, with respect to the monitoring laser light outputted from the back end face of the semiconductor laser element 106, the first optical detector 116 and the second optical detector 117 which receives the light which passes through the optical filter 115 detects a shift of wavelength. The first thermo-module 102 under the semiconductor laser element 106 is controlled, for example, in order to change temperature of the semiconductor laser element 106 to correct the shift of wavelength. The temperature of the semiconductor laser element 106 is adjusted by the first thermo-module 102 and feedback-controlled to suppress change of wavelength.
The laser light outputted from the front end face of the semiconductor laser element 106 is converted into a parallel right by the parallel lens 105 and connected with an optical fiber 121 by a condensing lens 120. The light transmitted in the optical fiber 121 is utilized for a desired use.